In patients with overweight or obesity who undergo total thyroidectomy, choosing the thyroid hormone replacement dose based on body weight is frequently inaccurate and could be responsible for prolongation of the titration period, according to study results published in Thyroid.

After total thyroidectomy, the initial dose of levothyroxine for thyroid hormone replacement therapy is usually based on body weight and the most common formula used is 1.6 µg/kg body weight, without correction for the patient’s body mass index (BMI). However, some studies have shown that patients with obesity have suboptimal response when hormone dosing is based on calculations developed for a non-obese population. Researchers in this study aimed to develop a proper dosing algorithm for levothyroxine in patients with obesity.

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The retrospective study included patients receiving thyroid hormone replacement after total thyroidectomy with no evidence of malignancy. The time to achieve euthyroidism was defined as the time from surgery to obtaining the first normal thyroid-stimulating hormone value.

The researchers reviewed charts of patients who underwent total thyroidectomy between July 2011 and December 2015. The final cohort included 114 patients (mean age, 55 years; 84% women) with final benign pathology and complete follow-up data who achieved euthyroidism.

The mean BMI of the study patients was 31 kg/m2 and approximately half had a BMI ≥30 kg/m2, including 23 patients (20%) with a BMI of 31 to 35 kg/m2, 19 (17%) with a BMI of 35 to 40 kg/m2, and 13 (11%) with a BMI >40 kg/m2. Patients with a higher BMI required a higher absolute levothyroxine dose (µg; P <.01) but the dose was significantly lower in relation to weight (µg/kg; P <.01).

The levothyroxine dose required to achieve euthyrodism according to BMI was: BMI <25 kg/m2, 1.76 µg/kg; BMI 26 to 30 kg/m2, 1.47 µg/kg; BMI 31 to 35 kg/m2, 1.42 µg/kg; BMI 35 to 40 kg/m2, 1.27 µg/kg; and BMI ≥40 kg/m2, 1.28 µg/kg.

The mean time to achieve euthyroidism was 50 weeks, with no statistically significant difference between the BMI categories (P =.58).

The researchers acknowledged several study limitations, including its retrospective design, data collection from electronic medical records, and no available data on compliance with treatment.

“[W]e recommend using either the actual weight of the patients with adjustment of dosing based on the BMI or using the adjusted body weight without regard to the patients BMI,” concluded the researchers.

Reference

Papoian V, Ylli D, Felger EA, Wartofsky L, Rosen JE. Evaluation of thyroid hormone replacement dosing in overweight and obese patients after a thyroidectomy [published online September 6, 2019]. Thyroid. doi:10.1089/thy.2019.0251